An epoxy resin is excellent in processability, and various cured product characteristics, such as high heat resistance, high insulation reliability, high rigidity, a high adhesive property, and high corrosion resistance, are brought out of the resin. Accordingly, the resin has been used in a large amount in various applications, such as an electrical insulating material (casting, impregnation, a laminated plate, or a sealing material), a matrix resin for a composite material like a CFRP, a structural adhesive, and a heavy duty coating.
In contrast, an epoxy resin cured product has a low rupture elongation property, low fracture toughness, and low peel strength. Accordingly, in the application of a matrix resin for a composite material and the application of a structural adhesive where those characteristics are required, the characteristics have been improved by various kinds of modification, such as rubber modification and polyurethane modification.
In each of Patent Literature 1 and Patent Literature 2, there is a disclosure that an epoxy resin composition obtained as described below has high shear strength, high peel strength, and high torsional shear strength, and is excellent in adhesive property and impact resistance, and is hence suitable for a structural adhesive for an automobile. In a bisphenol A-type epoxy resin containing a hydroxyl group, a polypropylene diol and isophorone diisocyanate are blended so that the molar ratio “NCO/OH” of NCO groups in isophorone diisocyanate to the total of the OH groups of the bisphenol A-type epoxy resin and the polypropylene diol may be 1.0. A polyurethane-modified epoxy resin thus synthesized is blended with a specific epoxy resin, such as a polyoxyalkylene diglycidyl ether, to provide the composition.
However, in each of the literatures, there is no disclosure that resin characteristics and cured product characteristics are controlled by specifying the loading concentration of the epoxy resin containing a hydroxyl group at the time of the synthesis of the polyurethane-modified epoxy resin. In addition, in each of the literatures, there is no disclosure of data on the viscosity of the composition, and the rupture elongation, fracture toughness, and glass transition temperature of a cured product thereof.
In Patent Literature 3, there is a disclosure that a resin composition containing a urethane-modified epoxy resin obtained as described below serves as a cured product which has a high fracture toughness value and is useful in electrical and electronic applications, and building material applications. In a bisphenol A-type epoxy resin, a specific diol compound and diphenylmethane diisocyanate are loaded and caused to react with each other to provide a urethane prepolymer. After that, 1,4-butanediol serving as a chain extender is loaded to polyurethanate the prepolymer.
However, also in the literature, there is no disclosure that resin characteristics and cured product characteristics are controlled by specifying the loading concentration of the epoxy resin containing a hydroxyl group at the time of the synthesis of the urethane-modified epoxy resin. In addition, in the literature, there is no disclosure of data on the viscosity of the composition and the rupture elongation of a cured product thereof. In the literature, there is a disclosure of data on the fracture toughness and glass transition temperature of the cured product, and a significant improving effect on the former is observed. However, the latter is low for an epoxy resin cured product and hence heat resistance is not sufficient.